netfilter: nf_nat: make find/put static

The functions nf_nat_proto_find_get and nf_nat_proto_put are
only used internally in nf_nat_core. This might break some out
of tree NAT module.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>